1. Field of the Invention
The present invention relates to an improved process for producing a billet of powdery alloys to be used for plastic working of the powdery alloys.
2. Description of the Prior Art
In the conventional processes for the production of a shaped article from a powdery alloy by plastic working, the powdery alloy was packed in a can having a prescribed capacity, uniformly compacted and degassed to give a billet, which was heated if necessary and subjected to plastic working, such as extrusion, forging or rolling, to produce a worked article.
The extrusion in the aforestated prior art will now be described by way of example. In the initial stage of the extrusion, the extrusion pressure is at its maximum but is lowered upon the commencement of the extrusion of a material. Although a powdery alloy which is easily subjected to extrusion has been made into a satisfactory billet by the conventional process, the use of a powdery alloy difficult of extrusion, such as a rapidly solidified powdery alloy, for achieving a high quality, such as high strength or high hardness, makes the initial extrusion pressure extraordinarily high, thus limiting the working on an ordinary plastic working machine (extruder) or a die. In addition,the working temperature raised for facilitating the plastic working has caused the problem of deterioration of the mechanical characteristics of the billet to be produced, such as strength and hardness, owing to a change in the properties of the rapidly solidified powdery alloy. Moreover, a powder usually produced by the atomizing process is not uniform and, thus, classified on a classifier into fine powder and coarse powder, the former being used for producing a high-grade product, while the latter for a low-grade product or being discarded.
Further, after being packed into a can, the powdery alloy is usually subjected to heating, degassing and compacting in a hot press. However, there is the disadvantage that the compacted powder is adulterated with a gas such as air when taken out of the hot press for carrying out plastic working.
In order to overcome the above-mentioned disadvantage, there has heretofore been adopted a method, as shown in FIG. 10, which comprises packing a powdery alloy 12 in a can 11, sealing hermetically the can with a lid 13 having an exhaust gas pipe 14 for degassing by welding 15 or the like, degassing the powdery alloy through the exhaust gas pipe 14 while heating the billet, and flattening the exhaust gas pipe 15 after the completion of degassing to prevent a gas such as air from returning into the billet. However, the welding of the lid after the packing of the powdery alloy raises the temperature of the rapidly solidified powdery alloy in the can to a temperature higher than a prescribed value (for example, 400.degree. C.) owing to the heat of welding and, in some case, substantial deterioration in the mechanical characteristics such as strength and hardness occurs after working, thereby causing problems in practical applications.